Thermal expansion of lithiated silicon (Li₁₃Si₄ and Li₇Si₃) anodes: a powder neutron diffraction study

Whilst at room temperature structural changes on lithiation of the silicon electrode are hard to study due to formation of amorphous phases, at high temperatures, used in thermal batteries, clear phase changes linked to voltage plateaux are observed. Here we report results from a galvanostatic discharge of a FeS₂/LiCl–KCl/Li₁₃Si₄ cell at 500 °C. The cell discharge showed a flat voltage plateau between Li₁₃Si₄ and Li₇Si₃ which indicates that both phases are in equilibrium and are line phases. In situ powder neutron diffraction study of two lithiated phases of silicon, Li₁₃Si₄ and Li₇Si₃, was performed. The two phases were heated from room temperature to 500 °C. This showed the phases did not become amorphous and did not undergo phase changes, with both phases being remarkably stable. The variation with temperature of the unit cell parameters was linear. The volumetric thermal expansion coefficient of Li₁₃Si₄ is 99.3 × 10⁻⁶ °C⁻¹ and Li₇Si₃ is 106 × 10⁻⁶ °C⁻¹. The volumetric thermal expansion of the two phases is significantly larger than that of silicon and closer to the thermal expansion of lithium metal. Thus, the Li_xSi electrode is mechanically closer to lithium than to silicon, and it can be considered as silicon clusters embedded within the lithium array rather than the silicon lattice hosting lithium.